Various types of documents can now be duplicated thanks to the improvement in photocopiers. Even color documents, photos, and documents in which images are formed on various types of paper can be almost perfectly duplicated.
For example, a process after reading out a document is different between a case where a document in which an image is formed on a piece of normal paper is duplicated and a case where a document in which an image is formed on a piece of photo paper is duplicated. More specifically, performed after reading out a document is a process (an undercolor process) corresponding to the property of paper which is the base of a document. Color photocopiers preferably perform so-called undercolor removal by which, for example, the color of a document slightly tinged with yellow is converted to “white”. On this account, to perform an undercolor process, it is necessary to detect the undercolor level at the time of reading out a document.
In conventional color photocopiers, the detection of the undercolor level is typically carried out by pre-scanning (rough scanning which is carried out before the actual image scanning and is several times faster than the actual image scanning). By this pre-scanning, the type of the document (color or monochrome) and the undercolor level are detected. Therefore, in the actual image scanning for reading out an image, it is possible to start image output immediately after the completion of the image readout.
The aforesaid method for detecting the undercolor level by pre-scanning, however, has such a problem that the speed of duplication is slow because of a time required for pre-scanning.
To solve this problem, for example, there have been techniques proposed by Patent Document 1 (Japanese Laid-Open Patent Application No. 2000-232589; published on Aug. 22, 2000) and Patent Document 2 (Japanese Laid-Open Patent Application No. 1-296775; published on Nov. 30, 1989).
To be more precise, according to Patent Documents 1 and 2, an undercolor process is carried out based on image data obtained by the image scanning (original readout), without performing pre-scanning.
According to the technique disclosed in Patent Document 1, RGB tone data obtained by scanning a document is converted to Lab image data. The undercolor is detected based on the Lab image data, the Lab image data is converted to CMYK image data, and then an undercolor process is carried out using the CMYK image data.
According to the technique disclosed in Patent Document 2, the density histogram of RGB tone data obtained by scanning a document is figured out. Based on this density histogram, a predetermined tonal value is subtracted from the RGB data, so that an undercolor process is achieved.
The aforesaid conventional techniques, however, do not take account of the color reproducibility in a highlight, and have poor precision in undercolor removal.
More specifically, an undercolor process in Patent Document 1 is carried out using CMYK image data. Since CMYK image data is typically obtained by calculating natural logarithms of RGB image data, undercolor removal using CMYK image data is not precise.
In the meanwhile, according to Patent Document 2, since a particular proportion of RGB tone data is subtracted, undercolor removal is excessive in some cases.
In addition to the above, Patent Documents 1 and 2 merely carry out an undercolor process, without considering the color reproducibility in a highlight. In connection with this, CMYK image data is obtained by figuring out natural logarithms of RGB image data. On this account, in the highlight, even if RGB tonal values are greatly different between neighboring images, this difference between the images becomes not evident after the conversion to CMYK image data.
Meanwhile, Patent Document 3 (Japanese Laid-Open Patent Application No. 4-316273; published on Nov. 6, 1992) discloses an image processing apparatus which generates CMYK four-color signal in such a manner that black components of a target pixel and adjacent pixels are smoothed based on a black component extracted from CMY three-color image signal, and undercolor removal is carried out based on the smoothed black components.
Patent Document 4 (Japanese Laid-Open Patent Application No. 5-207280; published on Aug. 13, 1993; corresponding to U.S. Pat. No. 5,245,417) discloses a method of calculating correction data by approximating the detected undercolor level of a document by a linear function.
Patent Document 5 (Japanese Laid-Open Patent Application No. 8-186726; published on Jul. 16, 1996; corresponding to U.S. Pat. No. 6,064,494) discloses an image processing apparatus which detects the undercolor level of a document based on a density histogram, and corrects RGB image data supplied to image forming means in such a manner that the undercolor level of an image formed on a sheet by the image forming means is identical with a predetermined level.
Patent Document 6 (Japanese Laid-Open Patent Application No. 2004-201198; published on Jul. 15, 2004) discloses a method of carrying out undercolor removal in such a manner that a peak point based on which undercolor is determined is worked out, and correction to white (maximum tone) is performed if the peak point thus worked out is not white.
The color reproducibility in a highlight is not considered in these prior art documents, either.